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This paper shows that nanosecond repetitively pulsed discharges were able to mitigate the response of lean methane-air swirl flames to acoustic excitations, at pressures up to 3 bar. Flame transfer functions with and without plasma discharges were investigated at pressures of 1.2, 2.0, and 3.0 bar, in a frequency range 48–380 Hz. Results show that the plasma discharges decreased by up to 50% the gain of the flame transfer functions, regardless of the pressure. Mechanisms responsible for this effect are discussed. [ABSTRACT FROM AUTHOR]

This work proposed a kinetic model for RP-3, the most widely used military-civilian aviation fuel in China. Four hydrocarbons within the typical size of major components in RP-3, i.e., n-dodecane, iso-dodecane (2,2,4,6,6-pentamethylheptane), decalin and n-butylbenzene, were included in the component palette to improve the ability of the surrogate to mimic the properties related to the molecular weight. Seven properties, cetane number, molecular weight, H/C ratio, threshold sooting index, lower heating value, the proportion of -CH 3 and -CH 2 in the total carbons were selected as targets aiming at the comprehensive emulation of the chemical and physical propensities of RP-3. By sequential use of the genetic algorithm and a local search method, a surrogate containing 27.44% n-dodecane, 28.81% iso-dodecane, 26.12% decalin and 17.63% n-butylbenzene by mole was formulated. The autoignition delay times of the surrogate were measured using a heated rapid compression machine at pressures of 10, 15, 20 bar and equivalence ratios of 0.5, 1.0 and 2.0 over low-to-intermediate temperature range. Results show that the surrogate can not only emulate the target properties but also the key non-targeted properties. A kinetic model of 3065 species and 11,898 reactions was then developed based on the proposed surrogate to describe the chemical process during the combustion of RP-3. Simulations show that the model can predict the fundamental combustion datasets in the present work and literature satisfactorily, suggesting the rationality and applicability of the model. Rate of production and evolution histories of OH and HO 2 were then conducted using the kinetic model to provide insight into the combustion of RP-3. Analysis suggests that negative temperature coefficient behavior also exists in the first stage ignition. [ABSTRACT FROM AUTHOR]

As the product of transesterification between various oils and alcohols, biodiesel exhibits characteristics of renewability and a wide range of raw materials. The ignition delay times (IDTs) of methyl oleate (MEOLE) and methyl linoleate (MLINO), the primary constituents of biodiesels, were measured in a heated rapid compression machine (RCM) to investigate the impact of C C double bond on the reactivity and autoignition characteristics. The experiment was conducted under two pressures of 8 bar and 10 bar at an equivalence ratio of 0.3–0.8 and a temperature range of 800–1000 K. The qualitative investigation unveiled the influence of compressed pressure, equivalence ratio, and oxygen mole fraction on the IDT of MEOLE and MLINO under varying compressed temperatures. Besides, a comparison of the IDTs for two fuels with different unsaturation degrees under identical conditions revealed that MEOLE exhibited greater reactivity at low temperatures compared to MLINO, while the increase in unsaturation degree can moderate and enhance reactivity at high temperatures. Moreover, three kinetic models in the literature were validated against the newly measured IDTs. The results indicate that the detailed lumped kinetic model exhibits better predictive capacity compared to other models, but it still underestimates the reactivity of MEOLE and MLINO within the investigated temperature range. Subsequently, the detailed kinetic model was optimized based on sensitivity analysis, resulting in excellent agreement between the predicted values of the optimized model and experimental data. Furthermore, accurate predictions were made regarding the relative reactivity changes of MEOLE and MLINO with temperature. Finally, different types of experimental data in the literature were employed to validate the optimized model, yielding satisfactory results. [ABSTRACT FROM AUTHOR]

An in-depth study of the chemical kinetic mechanism of biodiesels can facilitate accurate prediction of engine combustion and emission characteristics. Surrogate fuel is a feasible method for simulating the combustion chemistry of real biodiesel. Methyl myristate (MMY), one of the main components and a widely used surrogate for biodiesel, was investigated for the autoignition characteristics using a newly heated rapid compression machine (RCM). In this work, the ignition delay times (IDTs) of MMY were measured at compressed pressures of 10/15/20/30 bar with compressed temperatures ranging from 700 K to 900 K, and equivalence ratios varying from 0.3 to 1.25. The experimental observation successfully captured a typical negative temperature coefficient (NTC) phenomenon of IDT. The present experiment is the first one to measure the IDTs of MMY in a RCM, while also marking the first instance of observing the NTC phenomenon in macromolecular fatty acid methyl esters above C10. Besides, a skeletal model for methyl myristate has been developed based on the idea of decoupling methodology. In terms of fuel-dependent reactions, the kinetic model is divided into two parts: high-temperature reactions and low-temperature chain branching reactions. Combined with the experimental and simulation results, the dependence of the IDT of methyl myristate on compressed temperature, compressed pressure, equivalence ratio, and oxygen mole fraction at low-to-intermediate temperature was systematically investigated. The simulation results show that the developed skeletal model captures the ignition delay times well at low-to-intermediate temperatures and accurately predicts both the NTC phenomenon and the corresponding temperature range. Based on this, the key reactions in the combustion of MMY under different conditions were elucidated through sensitivity analysis. Ultimately, the model validation was conducted against the high-temperature IDT data from the literature. The chemical kinetic models of various biodiesels are essential for simulating the working process of internal combustion engines and accurately predicting the combustion and emission characteristics. Among these, the autoignition characteristic is one of the most important parameters for model validation. Besides, the carbon chain length of methyl myristate closely resembles that of real biodiesels, making it an ideal surrogate fuel for biodiesels. In this study, the IDTs of MMY were measured for the first time at low-to-intermediate temperatures in a heated RCM, thereby experimentally confirming the NTC phenomenon of methyl myristate. Additionally, a skeletal model of MMY was developed based on the idea of decoupling methodology, which can accurately predict the autoignition characteristics of MMY at low-to-intermediate temperatures. The simulated values demonstrate good agreement with experimental data. Overall, the present study provides an IDT database for the development and validation of the methyl myristate mechanism. [ABSTRACT FROM AUTHOR]

Ammonia (NH 3) blending combustion strategy has been regarded as a high-efficient form of NH 3 utilization. To understand the high-pressure gas-phase auto-ignition characteristics of NH 3 blending with diesel, the ignition delay times of NH 3 /diesel mixtures with high NH 3 energy fractions of 70%, 80% and 90% were measured in an ultrahigh-pressure rapid compression machine, at high pressures of 50–120 bar, temperatures of 755–996 K, and equivalence ratios of 0.5–1.5. As a consecutive study of Part 1 (Combust. Flame, 251 (2023), 112391), this study mainly focuses on the auto-ignition characteristics and oxidation mechanism of NH 3 /diesel mixtures at pressures higher than 50 bar. The ignition-promoting effect of increasing pressure from 50 to 120 bar was experimentally confirmed. Moreover, the increase of the diesel energy fraction, equivalence ratio, and oxygen concentration was found to decrease the ignition delay time of the NH 3 /diesel mixtures. The chemical reaction mechanism proposed in Part 1 was utilized to simulate the auto-ignition of the NH 3 /diesel mixtures. Simulation results show that the mechanism was able to quantitatively capture the ignition delay times of the NH 3 /diesel mixtures at the wide pressures. The cross-reactions between NH 3 and diesel, especially reaction RH + NH 2 = R + NH 3 , was found to play a key role in the correct prediction of auto-ignition at high pressure. In addition, it is revealed the effect of pressure is closely related to OH production reactions 2OH (+M) = H 2 O 2 (+M) and NO + HO 2 = NO 2 + OH, the rate of which increase with rising pressure. The reaction H 2 NO + O 2 = HNO + HO 2 is an important pathway where oxygen concentration affects NH 3 /diesel mixture auto-ignition. [ABSTRACT FROM AUTHOR]

With the extensive exploration of advanced combustion modes and the increasing demand for engines with extremely high efficiency and low emissions, biodiesel has garnered significant attention and research as a renewable fuel source that boasts a wide range of raw materials. The ignition delay times (IDT) of methyl stearate, one of the main components in biodiesel, were measured in a new heated rapid compression machine (RCM). The data were obtained for the equivalence ratio of 0.3–0.8 and the compressed temperatures ranging from 820 K to 1000 K under three different pressures of 8, 10, and 12 bar. The influence of compressed temperature, compressed pressure, equivalence ratio, and oxygen mole fraction on ignition delay times was systematically discussed under experimental conditions, revealing the autoignition characteristics of methyl stearate in intermediate temperatures. Besides, the CRECK model and a skeletal model were verified against the experimental results, revealing that the model obtained by analogy significantly overestimates the low-temperature reactivity of methyl stearate. Furthermore, appropriate adjustments were made to the pre-exponential factors for the low-temperature elementary reactions in the CRECK model based on sensitivity analysis. The results indicate that the present model with optimization can well capture the dependence of the IDTs on the operating parameters under the investigated conditions, and the simulation results are in satisfactory agreement with the experimental data. Finally, the optimized model was verified against the experimental data from both the shock tube and jet-stirred reactor. To the authors' knowledge, this study represents the first gas-phase RCM experiment conducted on pure methyl stearate, which is a waxy solid at room temperature. [ABSTRACT FROM AUTHOR]

Ammonia (NH 3) is considered as a good carbon-free hydrogen-carrier fuel and has gained extensive attention as a promising alternative fuel for internal combustion engine in recent years. To explore the application potential of the NH 3 burned with diesel fuel in internal combustion engine, the auto-ignition delay times of NH 3 /diesel fuel blends were measured in a rapid compression machine at a wide NH 3 energy fraction of 20%, 40%, 60%, 70%, 80%, and 90%, temperature range of 675–995 K, pressures of 20–50 bar, and equivalence ratios of 0.5–1.5. According to the variation of ignition delay times with temperature, three different combustion regimes for NH 3 /diesel mixtures can be determined. It is found that the typical NTC behavior and two-stage ignition process were only observed at the regime where diesel chemistry dominates. Both the first-stage and the total ignition delay times increase with rising NH 3 energy fraction, but decrease with the increase of equivalence ratio. Then, an updated NH 3 /diesel kinetic mechanism was proposed by adding new cross reactions between diesel and NH 3. Results show that the current mechanism exhibits an obvious improvement for ignition delay time prediction compared to the original mechanism, though it still fails to reproduce the ignition delay times of NTC range for the mixture containing 20% NH 3. Further sensitivity analysis and the OH radical rate of production analysis indicate that the overestimation of the reactivity for the NH 3 /diesel fuel blends at NTC range is closely related to reaction NO+HO 2 NO 2 +OH. The promoting effect of NO species on the conversion of inactive HO 2 radical into active OH radical at low temperature greatly accelerates the autoignition. In addition, the current mechanism does not include the cross reaction between the large NTC-related species (QOOH, OOQOOH, OQOOH, ROO) and the N-containing species (NO, NO 2 , NH 2), especially for the NO related reactions, which may have a great impact on the IDT simulation of the NH 3 /diesel mixtures. Consequently, this work presents new ignition delay time measurements and mechanism optimization for NH 3 /diesel mixtures. Continuous works should be emphasized on these unclear reactions to further understand the combustion behavior of NH 3 /diesel fuel blends. [ABSTRACT FROM AUTHOR]

Autoignition characteristics of methyl palmitate (C 17 H 34 O 2), a waxy solid at room temperature, were investigated in a newly developed heated rapid compression machine. Ignition delay times (IDT) of methyl palmitate were firstly measured at the low-to-intermediate temperatures of 780–980 K and equivalence ratios of 0.3–1.0 under three pressures of 10, 15, and 20 bar. A negative effect of the increase in compressed pressure, equivalence ratio, and oxygen mole fraction on IDTs at different compressed temperatures was qualitatively and quantitatively revealed. Besides, the dependence of IDTs on compressed pressure and equivalence ratio decreases with increasing temperature. A recently optimized mechanism was used to predict the experimental results. The simulations show an obvious negative temperature coefficient (NTC) behavior at the compressed temperatures of 780–830 K which was not observed in the experiment. Meanwhile, the mechanism pronouncedly underestimated the reactivity of methyl palmitate at higher temperatures in this study. Sensitivity analyses at different temperatures and pressures for equivalence ratios of 0.5 were conducted to determine the crucial reactions that caused the difference between the simulation and experiments. On this basis, further optimization was carried out and the simulation results of the latest mechanism show a good agreement with the experimental data. It is verified against shock tube experimental data that the optimization adjustment in this paper has a small positive impact on the prediction ability of the originally documented mechanism under temperatures above 1000 K. In the end, validation for the optimized mechanism against experimental data in a jet-stirred reactor (JSR) was performed. Overall, the experimental results fill in gaps in IDT data on methyl palmitate at low-to-intermediate temperatures and provide a reference IDT database for the development and validation of methyl palmitate mechanisms. [ABSTRACT FROM AUTHOR]

Burning oxygenated hydrocarbon biofuels in engines is a viable path for saving energy and reducing carbon emissions. N-butanol and biodiesel are two representative biofuels and have attracted widespread interest. In this study, the blends of n-butanol and biodiesel (a waste cooking oil) with different n-butanol ratios (40%, 60%, 80% by volume) were adopted to study their autoignition characteristics in a newly developed ultrahigh-pressure rapid compression machine. The ignition delay times of the blends were precisely measured under wide pressures of 10/20/40/60 bar, equivalence ratios of 0.3/0.5/1.0, and a temperature range of 700–970 K. Experimental results show that the ignition delay time decreases with the increase of pressure and equivalence ratio at the investigated conditions regardless the blending ratios. It is also found that the ignition delay time becomes slightly longer with the increasing n-butanol ratio in the blends at temperatures below 820 K. However, as the temperature further increases, the ignition delay times of different blends get closer and has a crossover tendency. The composition of the biodiesel was quantitively analyzed and a surrogate fuel was developed. An optimized mechanism was proposed based on a documented detailed mechanism with 461 species and 18,217 reactions. Simulation results show that the optimized mechanism better captures the dependence of the measured ignition delay times on temperature, pressure, and blending ratios over the entire temperature range compared to the original mechanism. In the end, species evolution and sensitivity analysis were performed sequentially with the optimized mechanism to give kinetics insight into the chemical interaction between biodiesel and n-butanol. The experimental data and modeling results reported here provide a basis for understanding the combustion reaction kinetics of biodiesel/n-butanol blending fuels. [ABSTRACT FROM AUTHOR]

Natural gas/diesel dual-fuel (DF) combustion technology has attracted substantial attention in terms of thermal-efficiency improvement and emission reduction in advanced compression ignition engines. As known, autoignition behavior of DF mixtures plays a significant role in controlling the ignition timing and combustion performance in engines. In the present study, ignition delay time (IDT) measurements of methane/diesel mixtures with three varying diesel substitution ratios (denoted as DSR30, DSR50, and DSR70) were conducted on both a heated rapid compression machine and a heated shock tube under wide-range conditions (T = 640–1450 K, p = 6–20 bar, ϕ = 0.7, 1.0, and 2.0 in 'air' mixtures). Experimental results show that DF blends exhibit typical two-stage autoignition characteristics along with the negative temperature coefficient (NTC) response at the temperature region currently investigated, owing to the addition of high-reactivity diesel fuel. Moreover, both the total and first-stage IDTs decrease with the rise of pressure and equivalence ratio as well as diesel substitution ratio. Additionally, a crossover of IDTs occurs at a high temperature (~1400 K) for varying equivalence ratios, and there exists a non-linear promoting effect of diesel contents on IDTs. The simulation results performed using a detailed chemical kinetic mechanism (POLIMI_1412) in conjunction with a well-validated tri-component diesel surrogate show generally good agreement with the experimental results at the current test conditions. Furthermore, species evolution assisted with brute-force sensitivity analysis was carried out to further understand the autoignition chemistry of the DF mixture, particularly the chemical interplay between methane and diesel during the low-temperature ignition processes. It is found that methane hardly generates OH radicals, which are mainly produced via the low-temperature oxidation pathways of diesel fuel. The competition between methane and diesel for OH radicals inhibits the consumption of diesel while promotes the depletion of methane, resulting in an inhibiting impact on the overall reactivity of the reaction system. What's more, the experimental data reported herein provide a foundation for the development of DF kinetic models with high accuracy and robustness. [ABSTRACT FROM AUTHOR]